In order to write data to media, a write head is typically used. Such a write head is generally part of a merged head that includes a head for writing and a head for reading data from the media. FIGS. 1A and 1B depict a side view and a top view, respectively, of a portion of a merged head including a conventional write head 10 and a conventional read head 30. The conventional write head 10 includes a conventional first pole (P1) 12 and a conventional second pole (P2) 20 that are separated at the front, near the air-bearing surface, by a conventional gap 18. Also shown is the hardbake layer 22 that is typically photoresist that is used to insulate the coils 16 of the conventional write head 10. The conventional P112 includes a conventional pedestal 14. The conventional read head 30 includes first shield (S1) 32, read sensor 34 and second shield 36. The conventional P112 typically lies above S2 and is separated from S2 by a gap 38.
Although the conventional write head 10 functions, one of ordinary skill in the art will readily recognize that the write head 10 is subject to thermal pole tip protrusion. Pole tip protrusion adversely impacts the reliability of disk drives because a protruding pole is more likely to contact the media during operation. In thermal pole tip protrusion, the top of the P112 and/or P220 expands upon heating. The shields S1 and S2 may also protrude upon heating. During use, the write head 10 is subject to an ambient temperature rise. In addition, the read sensor (not shown) is typically a magnetoresistive sensor. Current is driven through the sensor during use. In addition, a write current is driven through the coils 16 during use. The current in the read sensor, the write current in the coils and eddy currents in the core of the conventional recording head 10 also cause heating of the conventional write head 10. Different portions of the write head 10 have different coefficients of thermal expansion. Most portions of the conventional write head 10 have a higher coefficient of thermal expansion than the underlying substrate. As a result, different portions of the write head 10 expand different amounts, causing the write head 10 to distort. The P112 and P220, as well as the hard baked photoresist 22, have large positive coefficients of thermal expansion. As a result, the tips of P112 and P220 near the air-bearing surface expand and protrude outward. Moreover, the currents in the conventional write head 10 cause greater local heating in the region of the poles, exacerbating the protrusion of the poles P112 and P220.
In order to address protrusion of the poles, more recently developed recording heads (not shown) use a material having a coefficient of thermal expansion that is less than that of the substrate as part of the conventional recording head. Such materials include SiO2 or FeNi INVAR (an alloy, typically including Ni and Fe and having low thermal expansion coefficient). Thus, the material will expand less than the substrate upon heating. As a result, stress that tends to counteract the thermal expansion is placed on the poles. This should reduce the protrusion of the tips of the poles.
Although the use of SiO2 and FeNi INVAR may reduce protrusion of the pole tips, one of ordinary skill in the art will readily recognize that such materials are difficult to use. SiO2 has a poor thermal conductivity and elasticity. In addition, SiO2 tends to delaminate. As a result, SiO2 is difficult to use in a recording head. Similarly, FeNi INVAR has the desired properties only in a narrow composition range around sixty-five percent Fe. However, this composition coincides with the transition between fcc (face centered cubic) and bcc (body centered cubic) structures for FeNi INVAR. Because the FeNi INVAR having the desired properties is near this phase transition, it becomes difficult to obtain a stable form of FeNi INVAR having the thermal expansion properties. As a result, the reduction of protrusion of the pole tips remains problematic.
Accordingly, what is needed is a system and method for reducing the tendency of the poles of a recording head to protrude. The present invention addresses such a need.